Generally, a plurality of Cardan joints (cross shaft couplings) have been used in an automobile steering device. The Cardan joint is a non-constant velocity joint which is such that when the working angle increases, the rotational variation between the input and output shafts also increases. Since it is necessary to secure constant velocity nature by combining a plurality of Cardan joints, there is a problem that the degree of freedom of design of vehicles is impaired.
Accordingly, if fixed type constant velocity joints are used as shaft couplings for steering devices, constant velocity nature can be secured at any working angle, providing the advantage of increasing the degree of freedom of design of vehicles. The fixed type constant velocity joint comprises an outer ring having a plurality of curved track grooves in its spherical inner surface, an inner ring having a plurality of curved track grooves in its spherical outer surface, torque transmitting balls incorporated between the track grooves of the inner and outer rings, and a cage for holding the torque transmitting balls.
The center of curvature of the track grooves of the outer ring (the outer ring track center) is offset with respect to the center of curvature of the spherical inner surface of the outer ring, and the center of curvature of the track grooves of the inner ring (the inner ring track center) is offset with respect to the center of curvature of the spherical outer surface of the inner ring, axially by the same distance to opposite sides, whereby the ball tracks constituted by the track grooves of the outer and inner rings are in the form of wedges spread toward the opening side of the outer ring.
In this connection, this kind of fixed type constant velocity joint has clearances created therein between the track grooves of the outer and inner rings and the balls because of the request made for functional and processing reasons. When either the inner ring and or the outer ring is fixed with the joint in the neutral position and the other is moved, such clearances appear as axial clearances, radial clearances or circumferential clearances. The track clearances greatly influence the circumferentially (rotation backlash) between the inner and outer rings. The track clearances are inevitable to the fixed type constant velocity joint from the standpoint of processing allowance and assemblability, resulting in large rotation backlash, causing the fear that such rotation backlash deteriorates the steering feel for substantially straight travel of the vehicle or is the cause of generation of abnormal sounds. To solve this problem, Japanese unexamined Patent publication 2003-130082 proposes a fixed type constant velocity joint capable of eliminating or suppressing the rotation backlash by closing the track clearances by a preloading means installed in the joint.
The fixed type constant velocity joint disclosed in Japanese unexamined Paten publication 2003-130082 has ball tracks in the form of wedges spread toward the opening side of the outer ring, and in order to close the axial clearances created by track clearances, it has an arrangement wherein a pressing section for axially applying elastic pressing force is provided on the inner ring side, while a receiving section for receiving the pressing force from the pressing section is provided in the cage. And, elastic abutment between the pressing section and the receiving section presses the inner ring toward the opening side of the outer ring, thus producing axial relative movement between the two. The relative movement closes the track clearances through the balls, thus preventing rotation backlash. Depending on the setting of the axial clearance defined by the spherical clearance between the inner ring and the cage and depending on the axial clearance produced by the track clearances, however, there are cases where the axial clearance produced by the track clearances cannot be closed completely, making it difficult to prevent rotation backlash.